This invention relates to the operation of push-pull amplifiers. In particular, the invention discloses a circuit which provides automatic bias adjustment for such amplifiers.
Push-pull amplifiers have been well known in the art for some time. Examples of such amplifiers in the patent literature include U.S. Pat. No. 3,531,728 to W. A. Visher; U.S. Pat. No. 3,376,515 to W. G. Dilley; and U.S. Pat. No. 4,077,013 to G. S. Morez et al.
One problem common to all push-pull class AB amplifiers is that of correctly biasing the output stage for both minimum crossover notch distortion and low power dissipation. Conventional biasing circuits, even after manual trimming to compensate for component variations, do not always provide the desired bias level because of their sensitivity to changes in temperature. Thermal feedback techniques can greatly decrease this sensitivity but cannot effectively eliminate the problem in production because of the difficulty in achieving accurate tracking between output devices and thermal sensors. Other techniques to improve bias stability have adverse effects such as reduced open loop gain and increased power loss.
In any push-pull amplifier, there are two halves that make up the output stage. One provides positive current to the load; the other, negative. In addition, there is a current known as bias (also known as idling or quiescent current) which does not flow into the load but, instead, runs through both sides of the output stage to smooth the transition between positive and negative current excursions. There is an ideal level for this bias which will produce the best compromise between distortion and power efficiency in an amplifier.
In order to regulate the bias current, it is first necessary to measure it. While this task is easy to accomplish under amplifier idling conditions, it is difficult while the amplifier is active because the signal current, which is unpredictable and large in magnitude, follows nearly the same circuit path as the bias, obscuring any direct measurement. Fortunately, there is one feature of the push-pull amplifier configuration that permits measurement of bias current at certain times. Since signal current cannot flow through both halves of the output stage at once, then if both sides are simultaneously drawing more current than the desired bias level, it follows that the bias level must be too high. Similarly, if both sides of the push-pull amplifier are simultaneously drawing less current than the desired bias level, then the bias is too low. One of these two conditions will occur each time there is an output current zero crossing and the bias current is not equal to the desired level. By testing for these two conditions, a signal can be generated to correct for bias level deviations from the ideal. The latter signal can be fed back by appropriate circuitry to regulate the bias as necessary. The automatic bias adjusting circuit disclosed herein samples the bias level at output current zero crossing, and actively adjusts the bias current level to conform to the preselected value.